1. Field of Invention
The invention is directed to devices for milling a window in casing disposed in an oil or gas wellbore and, in particular, to four-mill bottom hole assemblies for cutting a window in the wellbore casing such as for allowing a lateral, offshoot, horizontal, or branch wellbore to be drilled.
2. Description of Art
Bottom hole assemblies, or casing window milling assemblies, for use with whipstocks disposed within wellbore casing are known in the art. In general, these assemblies operate by lowering the assembly into a wellbore casing until a cutting end, or mill head or window mill, contacts the whipstock. As the assembly is further lowered, the window mill is forced into the wellbore casing by the whipstock. As a result, the window mill begins cutting the wellbore casing to form a window.
Contemporaneously, two additional, or secondary, mills such as a reaming mill and a honing mill, begin cutting the wellbore casing above the window formed by the window mill. As the window mill moves further downhole, and is further forced into the wellbore casing by the whipstock, the opening in the casing, or window, is enlarged, usually by the two secondary mills cutting additional openings in the casing above the whipstock and gradually moving toward the window formed by the window mill until the openings and the window connect. To assist with the bending moment caused by the window mill being forced by the whipstock into the wellbore casing, a flex-joint or flexible section within the upper mills is usually disposed above the window mill.
Although prior assemblies are effective at ultimately forming the desired opening in the wellbore casing, they have several shortcomings. For example, the size of the window ultimately cut in the casing should, theoretically, be as long as the ramp of the whipstock. The length of the ramp of the whipstock is defined as the distance along the angled portion of the whipstock from the point where the window mill is first moved toward the casing wall to the bottom of the angled portion. However, the window formed by the typical three-mill bottom hole assemblies have difficulty cutting a window that is as long as the ramp length of the whipstock because of the loss of appreciable restraining force on the window mill during its traverse on the bottom quarter section of the whipstock ramp. As a result, the length of the window is shortened such that longer and larger diameter assemblies and other equipment which, in most cases, are more desirable, cannot pass through the opening.
Current casing window milling assemblies also experience problems with the cutting structure on the mills wearing out prematurely while cutting a window in large size casings with large size whipstocks. In many instances, three mills in three-mill assemblies do not ensure enough cutting structure to create a full gauge window while sustaining the long ramp lengths of large size whipstocks. The vibration impact can also cause the cutters to breakdown and the mills loose their cutting ability prematurely. This can lead to the considerable expense of a second milling operation with a fresh set of mills.
Also, in many situations, disposition of a full gauge secondary reaming/honing mill at a location too close to a full gauge window mill produces large bending stresses in the bottom hole assembly, especially between the window mill and the secondary mill.